NPTEL – Chemical Engineering – Nuclear Reactor Technology
Breeders as an Inexhaustible Energy
Source
K.S. Rajan
Professor, School of Chemical & Biotechnology
SASTRA University
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NPTEL – Chemical Engineering – Nuclear Reactor Technology
Table of Contents
1 NUCLEAR FUEL CYCLE ................................................................................................................ 3 1.1 OPEN FUEL CYCLE ...................................................................................................................................... 3 1.2 CLOSED FUEL CYCLE .................................................................................................................................. 5 2 REFERENCES/ADDITIONAL READING ................................................................................... 7 Joint Initiative of IITs and IISc – Funded by MHRD
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NPTEL – Chemical Engineering – Nuclear Reactor Technology
In this lecture we shall discuss the role of breeder reactors as a source of harnessing
nuclear energy for longer durations, making it virtually inexhaustible.
At the end of this lecture, the learners will be able to
(i)
realize the limited availability of uranium for use in a non-breeder reactor
(ii)
understand the role of breeders in increasing the uranium availability
An inexhaustible energy source is the one from which energy can be harnessed
indefinitely without any depletion or diminishment. Energy can be harnessed from
solar energy through solar photovoltaic cells or solar thermal and hence solar energy
is an excellent example for inexhaustible energy source.
In light water reactors, only a small fraction of the total uranium supplied is utilized
depending upon the level of fuel enrichment. Some method of supplying fresh fuel
while removing the spent fuel is followed to sustain the energy production. Escalation
in the price of uranium will increase the cost of power generation. Hence the cost of
energy production from light water reactors is dependent on the cost of uranium. The
fact that light water reactors require enriched uranium means that low quality or lowenrichment fuel is unsuitable for use in them. Though heavy water reactors can
generate power with natural uranium, the cost of energy production is still dependent
on the cost of uranium to be supplied from time-to-time. Hence for the nuclear power
generation to be sustainable economically for longer periods, the over dependence of
cost of energy production on cost of fuel must be brought down.
1 Nuclear Fuel Cycle
We will look at the ‘Nuclear Fuel Cycle’ to understand the way the fuel is handled
from its mining till its disposal. The fuel cycle comprises the processes carried out to
transform ore to the fuel suitable for use in core (called front end of the cycle),
transformations to the fuel while it is being used in the reactor (called service period)
and the activities performed to deal with the spent fuel from the reactor (called back
end of the cycle).
There are two types of fuel cycles: Open fuel cycle and Closed fuel cycle.
1.1 Open fuel cycle
The cycle begins with the extraction of uranium ore from mine followed by
processing the extracted uranium. Enrichment is carried out to increase the percentage
of U-235 from 0.7 % (natural uranium) to 2-3.5 % required for light water reactors.
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For use in heavy water reactors, enrichment is not required. Fuel is fabricated in the
desired form and loaded in the core. These constitute front end of the cycle.
Uranium ore from mine Ore Processing Enrichment for LWR Fuel Fabrication Power generation in reactor core Spent fuel containing fissile isotopes U-­‐235, Pu -­‐239 and fertile isotope U-­‐238 Disposal Fig. 1. Schematic layout of open fuel cycle
After the fuel has been irradiated for power generation (service life), the fuel from
core is removed as spent fuel. The spent fuel too contains fissile isotopes like U-235
and Pu-239 but in lower amounts that do not sustain chain reaction. Apart from fissile
isotopes, fertile isotopes, minor actinides and activation products are also present in
spent fuel. Safe disposal of spent fuel completes the cycle wherein no effort is made
to separate the components of spent fuel.
Note: Activation products are the products formed due to the neutron absorption by
non-fuel components of reactor like coolant, control rod and structural materials.
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1.2 Closed fuel cycle
The closed fuel cycle is similar to that of open fuel cycle during front end of the cycle
and service periods. The difference lies in the handling of spent fuel. While no
attempt is made to reprocess the spent fuel in open cycle, the same is reprocessed in
closed fuel cycle. The reprocessing is aimed at separating the plutonium (produced by
nuclear transmutation) and unutilized uranium from other components of spent fuel.
The recovered uranium and plutonium are recycled for use as fuel in nuclear reactors.
Hence through this cycle, better utilization of resources can be made. The products of
fission are separated and then sorted depending on their half lives and activities,
before disposing them appropriately without excessive burden to environment.
Uranium ore from mine Ore Processing Enrichment Fuel Fabrication Power generation in reactor core Spent fuel containing fissile isotopes U-­‐235, Pu -­‐239 and fertile isotope U-­‐238 Pu + U (fissile components) Reprocessing Disposal Fig. 2. Schematic layout of closed fuel cycle
Most light water reactors follow open fuel cycle, in which the spent fuel is disposed
of without any reprocessing (and not reused in reactor in any form) in accordance
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NPTEL – Chemical Engineering – Nuclear Reactor Technology
with the laws prevailing in the respective country. Appropriate precautions are taken
to store the spent fuel safely in an isolated region.
With the known conventional resources of uranium and with the number of light
water reactors in operation and those planned for future, uranium is expected to be
available for about 80 years only. With additional uranium resources still unexplored
this may extend to about 270 years.
Fast breeder reactors use closed fuel cycle. A typical core of fast breeder reactor
contains a mixture of UO2 and PuO2. While U-235 in UO2 and Pu-239 in PuO2 act as
fissile material generating energy, the abundant U-238 in blanket undergoes nuclear
transmutation producing more Pu-239. As a result the energy that can be produced
from uranium in a fast breeder reactor is significantly more than that produced in a
thermal reactor. On a conservative estimate, energy potential in fast breeder reactor is
about 60 times greater than that in a thermal reactor for the same initial uranium
inventory. If all the existing thermal reactors were replaced by fast breeder reactors, it
would be possible to utilize the known conventional resources of uranium to operate
these reactors for about 4500-5000 years (80*60), a time period high enough to
consider the source to be inexhaustible.
The higher energy potential in fast breeder reactor means that the power cost can be
independent of fluctuations in fuel cost. The fresh fuel to be replenished is minimal, if
not zero and hence makes it possible to de-link cost of power production from the
fluctuations in the price of fuel.
With the closed fuel cycle possible with fast reactors, the quantity of accumulated
long-lived isotopes that must be disposed is likely to be less than 0.1 % of the fission
products. This reduces the burden of waste management by reducing the storage space
required for disposal. Also the time for which the fuel needs to be stored to bring the
toxicity down to natural levels is reduced considerably, say by 1000 times. In
nutshell, with closed fuel cycle adopted in Fast Breeder Reactor, the following
advantages are envisaged:
(i)
(ii)
reduction in mining of fresh uranium
considerable reduction in waste management time and money
In summary, with the use of fast breeder reactor nuclear energy may be made an
inexhaustible source due to increased utilization of energy potential of uranium,
independence on fluctuations in fuel cost and lesser waste generation.
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2 References/Additional Reading
1. A.E. Waltar, D.R. Todd, P.V. Tsvetkov (Eds.), “Fast Spectrum Reactors”, Springer,
2012 (Chapter 1)
2. D. Bodansky, “Nuclear Energy: Principles, Practices, and Prospects”, 2/e, Springer,
2004 (Chapter 9)
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